There are many water treatment techniques using a granular bed adapted to assure either a conventional filtration of water or a filtration associated with a biological purification of used water, i.e. waste water. Because of the fact that there is produced over time a filling of the inter-granular spaces and therefore a clogging of the bed, each of these techniques consists in alternating water treatment cycles using the bed, and bed cleaning cycles for cleansing of the bed.
A first process thus consists during each treatment cycle, of causing the water to be treated to trickle through a completely emersed granular bed. This process however includes a major disadvantage resulting from the fact that the residence time of the water to be treated is relatively short, which leads to a mediocre effectiveness with respect to the elimination of soluble and particulate pollutants. On the other hand, the uncoupling of the biological film alters the quality of the treated effluent by the presence of materials in suspension.
To overcome this drawback, other processes have contemplated, during each treatment cycle, using a partially or completely submerged treatment bed comprised of granular support materials of a volumic mass greater than that of the water to be treated. These processes permit, in effect because of an increase in the residence time of the water to be treated, achieving a much more efficient biological purification than when the bed is completely emersed. Under these conditions, the retention of materials in suspension is favored but it leads to a clogging of the granular bed which is appropriate to control.
The cleaning cycles for the processes described above necessitate loosening or unpacking the granular bed by means of an air/water mixture in order to obtain a fluidization of this bed which is generally obtained by means of air, in order to conserve water, the water then being used for carrying off and recovering the sediment. The cleaning thus consists of loosening the granular bed by means of air, then evacuating the sediment by means of a continuous flow of water. The drawback of such a cleaning process resides in the fact that it requires a relatively significant water consumption which, in practice, turns out to correspond to a volume equal to several times the volume of the reactor, being able to represent up to 10% of the volume of water treated.
Another treatment technique has consisted in carrying out a filtration and a biological purification by means of a immersed floating granular bed, blocked on its upper portion by a grill, and comprised of support materials of a volumic mass much lower than that of the water, through which the water is caused to circulate in a rising or descending flow. According to this process, the cleaning of the granular bed is achieved according to the principle of a hydraulic flushing which permits an important saving of water with respect to the processes described above, while favoring an expansion of the support materials. However, such a cleaning often appears to be incapable of creating an adequate release of the sediment from the support materials, particularly during treatment of used water by a fixed culture, which frequently leads to an overall diminution of the effectiveness of the reactor.
The present invention seeks to overcome these aforementioned drawbacks of known processes, and has as its primary object to provide a process for the treatment of water in which the cleaning operations of the granular bed are optimized, that is requiring a very low volume of wash water, and leading to an effective unclogging of the granular bed, followed by a complete gravity separation of the sediment from the mixed liquor and the support materials thus permitting a complete evacuation of the sediment by a simple draining.
Another object of the invention is to provide a process permitting optimizing the retention of material in suspension and the oxidation of the soluble pollutants.